Soluble coffee and tea powders are usually prepared by extracting soluble solids from roast and ground coffee or tea leaves, concentrating the extract so obtained, and drying the extract to powder, either by spray-drying or freeze drying. To increase yields to commercially acceptable levels, the extraction is usually carried out in two or more steps. In the first step, readily soluble matter is extracted from the roast and ground coffee or tea leaves. In subsequent steps, insoluble carbohydrates remaining in the partially extracted grounds or leaves are solubilized using thermal solubilization and further extracted; either simultaneously or in separate solubilization and extraction stages.
The extraction is traditionally carried out in a battery of fixed bed reactors in a counter-current manner. An extraction liquid, usually hot water, is introduced into the fixed bed reactor which contains the most extracted coffee grounds or tea leaves and flows from one reactor to the next, leaving the battery after the reactor which contains the freshest grounds or leaves. After a certain time, the process is halted and the reactor which contains the most extracted grounds or leaves is disconnected from the battery. A reactor which contains fresh grounds or leaves is then connected to the opposite end of the battery and the process is repeated.
The traditional process works well but suffers the disadvantage that it is not continuous while the downstream production steps are substantially continuous. Further, the extracted grounds or leaves must be dug out or blown out of the reactor, which is removed from the battery, prior to the reactor being re-filled with fresh grounds or leaves. Also extraction and solubilization are essentially coupled in the traditional process. Therefore the solubilization conditions cannot be adjusted independently from the extraction conditions. This, along with high pressure drops over the fixed bed reactors, prevents the use of potentially advantageous solubilization conditions. Further, these problems often lead to the grounds or leaves having long residence times in the extraction process, which is detrimental to product quality and leads to tars which foul equipment. Therefore, there have been many attempts over the years to replace the traditional process with a continuous process and to decouple extraction from solubilization. None of these attempts have been overly successful.
One attempt is described in U.S. Pat. No. 2,629,663. In the process described in this patent, the coffee grounds are fed into a tubular reactor which contains an auger. The auger then transports the coffee grounds through the reactor. Extraction liquid is caused to flow through the reactor, counter-current to the coffee grounds. The reactor has several zone of differing diameter to prevent plugging of the reactor due to swelling of the grounds. A problem with this process is that the mechanical action of the auger on the coffee grounds is likely to generate fines. Also low extraction temperatures are used and there is no solubilization. The process is unable to produce products of commercial interest today.
Another attempt is disclosed in U.S. Pat. No. 3,862,347. In the process described in this patent, coffee grounds are slurried with a concentrated coffee extract and introduced into the bottom of a tubular reactor. A cold, dilute coffee extract is introduced into the top of the tubular reactor as an extraction liquid. Hollow blades rotate in the tubular reactor near its bottom. Extraction liquid travelling down the reactor is drawn off through the blades. The blades also serve to transport the coffee grounds upwards. Once the coffee grounds are transported above the hollow blades, the grounds form a packed bed which moves upwards against the flow of the extraction liquid. The temperature within the bed is carefully controlled so that the viscosity of the extraction liquid decreases or remains the same as it travels down through the bed. However, the process described in the patent is an attempt to mechanically optimize the system by reducing channeling. The process does not improve the flavor of the extracted coffee. The arrangement of column and the blades is such that the ratio of the height of the packed bed to the diameter of the column is above about 7:1. Hence the residence time of the extract in the column is long which ordinarily negatively affects flavor. Also, pressure drops across the bed are likely to be high.
It is an object of this invention to provide a process for the continuous extraction of water soluble substances from solids which contain them, especially to provide a beverage products of good flavor.